X. STARFISH, SEA URCHIN, BRITTLE STARS 



Branch IX. — Echiiioderuiata (er^uios (spine), hedge- 

 hog; derma, skm) 



Most of the meml^ers of this Ijranch have the surface of 

 the body beset with spines; hence the name, Echinoder- 

 mata, meaning spin)' skins. In all the echinoderms the parts 

 of the body are arranged in a radial jnanner. In nearly 

 all there is an exoskeleton composed of calcareous plates. 

 Without exception they are found in the sea. 



An Example of the Branch — the Starfish 



The form and radial symmetry of the starfish, — The 



common starfish has a star-shaped body consisting of a 

 central portion kno^\'n as 

 tiie central disk, and Uxo 

 tapering arms. The arms 

 radiate from the central disk 

 much as the spokes of a 

 wheel radiate from the hub. 

 This radial arrangement is 

 characteristic of all mem- 

 bers of this branch and is 

 known as radial synunetry 



(Fig. 43). 



External features. —In its natural position, one side 

 of the body is always uppei-most. This side is called the 



Fig. 43. — Vt-iitral surface of a 

 starfish. 



